Adding a product like that to lower the pH is not a good idea. It is unlikely to lower the pH except possibly temporarily, so you would be constantly adding more and more. And pH is not a critical parameter. If you are using your tap water you can assume the fish store where you got the fish is also using that tap water, so the fish are acclimated to the water. Finally, many water companies add something to the water to raise the pH to protect the copper pipes in people's homes from eroding away, but that usually only temporarily raises the pH. If you let the water stand for several hours the pH will probably drop.